The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Existing prior art UPS load sharing systems are typically designed to meet one of two important criteria, but not both. The first is that the load sharing system needs to provide an effective way to share load between UPS modules so that the failure of one UPS module does not allow any load in the system to be dropped. The second is that the load sharing system be designed so that it can provide an effective means of isolating UPS modules from each other, to thus ensure that a fault on the electrical distribution bus which the UPS modules are coupled to will not allow any load to be dropped. However, as noted above, present day load sharing systems cannot provide both of these features at the same time.
The above described limitation is illustrated in the prior art load sharing system shown in FIG. 1. In any load sharing system there must necessarily be a “common” electrical bus which provides the path for current to flow between UPS modules and their respective loads. There may also be a number of reactive impedance components connected between the UPS modules and the common bus. The reactive components allow current to flow therethrough based on an AC voltage phase difference between the UPS modules which is controlled by a well-known and understood “frequency droop” method of power load sharing between AC sources. In prior art load sharing systems, this common bus is a single electrical bus with an inductor connected between each of the UPS modules and the common bus. The inductor can accomplish two things. Firstly, in the event of a fault on the common bus, if the inductor impedance is large enough, it can provide sufficient isolation between the UPS modules so that the fault current does not cause the UPS to exceed its current limit and lose the output voltage to the load. Secondly, in the case of a failed UPS module that goes off line, if the inductor impedance is small enough, it can allow sufficient current flow from the remaining UPS modules to the failed UPS module bus to support the load. The unfortunate limitation of this system is that it cannot provide both features at the same time. If the inductor is large enough to provide the isolation required during a common bus fault, then its impedance will be too large to allow the current flow needed to support the load of a failed UPS module. In fact, there is approximately a 2 to 1 difference between the two desired inductor values. The user of this system must therefore decide which feature is more important. The only alternative with the prior art load sharing system of FIG. 1 is to severely limit the UPS load to a point where both features can eventually be obtained. But this choice comes at a great cost penalty since the UPS modules must be used far below their rated capacity.
A new load sharing system which provides both the ability to isolate the UPSs in the event of a fault on the common bus, as well as to still enable a load to be powered when a given one of the UPSs associated with the load fails, provided that the overall power capacity available from the remaining UPSs is not exceeded, would thus provide both of the above described important features in one load sharing system.